Measurement of blood flow in the heart and vessels using the Doppler effect is well known. Whereas the amplitude of the reflected waves is employed to produce black and white images of the tissues, the frequency shift of the reflected waves may be used to measure the velocity of reflecting scatterers from tissue or blood. Color flow images are produced by superimposing a color image of the velocity of moving material, such as blood, over the black and white anatomical image. The measured velocity of flow at each pixel determines its color. The process by which by which black and white images are formed is conventionally referred to as B-mode imaging, while the process by which flow velocity is imaged using colors is conventionally referred to as color flow imaging.
The present invention is incorporated in an ultra-sound imaging system consisting of four main subsystems: a beamformer 2 (see FIG. 1), processors 4, a scan converter/display controller 6 and a master controller 8. System control is centered in the master controller 8, which accepts operator inputs through an operator interface (not shown) and in turn controls the various subsystems. The master controller also generates the system timing and control signals which are distributed via a system control bus 10 and a scan control bus (not shown).
The main data path begins with the analog RF inputs to the beamformer from the transducer. The beamformer outputs two summed digital baseband receive beams. The baseband data is input to the processors 4, where it is processed according to the acquisition mode and output as processed acoustic vector (beam) data to the scan converter/display processor 6. The scan converter/display processor 6 accepts the processed acoustic data and outputs the video display signals for the image in a raster scan format to a color monitor 12. The scan converter/display controller 6, in cooperation with master controller 8, also formats multiple images for display, display annotation, graphics overlays and replay of cine loops and recorded timeline data.
The B/M processor converts the baseband data from the beamformer into a log-compressed version of the signal envelope. The B function images the time-varying amplitude of the envelope of the signal as a grey scale. The envelope of a baseband signal is the magnitude of the vector which the baseband data represent. The phase angle is not used in the B/M display.
The frequency of sound waves reflecting from the inside of blood vessels, heart cavities, etc. is shifted in proportion to the velocity of the blood cells: positively shifted for cells moving towards the transducer and negatively for those moving away. The Doppler processor computes the power spectrum of these frequency shifts for visual display and it also synthesizes an audio signal from the separated positive and negative shifts.
The color flow (CF) processor is used to provide a real-time two-dimensional image of blood velocity in the imaging plane. The blood velocity is calculated by measuring the phase shift from firing to firing at a specific range gate. Instead of measuring the Doppler spectrum at one range gate in the image, mean blood velocity from multiple vector positions and multiple range gates along each vector are calculated, and a two-dimensional image is made from this information.
The acoustic line memory 14 of the scan converter/display controller 6 accepts processed digital data from the processors 4. The scan converter 16 performs the coordinate transformation of the color flow and B mode data from polar coordinate (R-.theta.) sector format or Cartesian coordinate linear array to appropriately scaled Cartesian coordinate display pixel data stored in X-Y display memory 18. The X-Y display memory 18 provides storage for up to three X-Y image frames.
The M mode and Doppler data types are interpolated in both dimensions (range and time for M or frequency and time for Doppler) by the timeline processor incorporated in the timeline/graphics processor and display memory board 20. The graphics data for producing graphics overlays on the displayed image is also generated and stored on the timeline/graphics board 20. The video processor 22 displays the resulting image in a raster scan format on video monitor 12.
Timeline displays provide a representation of the vector data acquired along a particular scan line multiple times over time. In contrast, the X-Y image frame data is derived from the vector data acquired along multiple scan lines. The rate at which new vector data is obtained along a particular scan line is greater than the acoustic frame rate in the ultrasound imaging system shown in FIG. 1.
For B mode images, the acoustic line memory 14 acquires and stores the baseband data in a polar or Cartesian vector format from the B/M processor. The scan converter 16 generates addresses used to map the information into a pixel value at a given X-Y coordinate for display. The mapping function utilizes a two-dimensional interpolation. The acoustic line memory 14 performs the same function for color flow images.
For M mode images, the timeline/graphics board 20 acquires, interpolates and stores the M mode pixel values for raster display and review in a moving bar format. For Doppler data, the timeline/graphics board 20 acquires Doppler spectra data and overlay data. The Doppler spectra data is interpolated and stored as pixel values for raster display and review. The overlay data is interpolated in time and converted to overlay graphic data for raster display.
During timeline vector acquisition, an update bar appears at the left side of the display and begins to move to the right at a previously commanded sweep speed. Live imagery appears to the left of the moving bar and remains stationary. Any previously displayed recorded image to the right of the moving bar is erased as the bar covers it.
The video processor 22 multiplexes between the graphics data, image data, and timeline data to generate the final video output. Additionally it provides for various greyscale and color maps as well as combining the greyscale and color images.
The cine board 24 provides resident digital image storage for single image review and multiple image loop review and various control functions. The region of interest displayed during single-image cine replay is that used during the image's acquisition. The cine memory also acts as a buffer for transfer of images to digital archival devices via the master controller 8.
Ultrasound images, which are produced at a rate of 30 frames per second for a 60-Hz system, are captured as X-Y data in cine memory. Graphics data (in RGB format) may be associated with each image frame on the display, but it is not captured in cine memory along with the image. The graphics data associated with each frame (such as time, the color flow region of interest, and Doppler and M cursors) can change from frame to frame.
The timeline/graphics board 20 also provides a review of previously acquired timeline vector data. The timeline replay is presented as a scrolling display, with the entire review portion of memory 20 appearing as one continuous strip. The operator controls the scroll with a trackball, rolling the trackball to the right to scroll the display to the right to display the timeline data recorded earlier in time, until the end of the timeline replay memory is reached.
FIG. 2 shows a series of image frames as their associated Doppler cursor sweeps across the image from left to right. The image data stored in cine memory will contain image frame 0 through image frame 30, but it will not contain the graphics data Doppler cursor. During cine replay, current ultrasound implementations use one of two methods to display graphics.
FIG. 3 shows the cine replay of the data captured in FIG. 2 in accordance with a first conventional method. Since the graphics data is not captured with the image data, the graphics associated with the last captured cine frame is left on the display during the entire cine replay. In the example shown in FIG. 3, the Doppler cursor graphic associated with image frame 30 (the last captured cine frame, see FIG. 1) remains on the screen.
In accordance with a second conventional method, any change to an image graphic (such as the color flow region of interest, or the Doppler and M cursor) will result in a cine dump (i.e., the contents of the cine memory are flushed or cleared). This method ensures that the graphics always match the image data. This method is often viewed as restrictive by users.